In commercial cellular wireless systems (for example, those based on the well-known 3GPP LTE standard for 4G wireless communication) user devices and base stations are designed to support communication links where the relative speed between a base station and a user device can be, for example, up to 400-500 km/hour. The greater the relative speed between a user device and a base station, the larger the well-known Doppler shift that will occur during a transmission from one to the other.
A user device can be a cell phone or other similar device and a base station can be a cell tower or other similar device. Generally, the base station is stationary and the user device is mobile, but it is possible that either or both the user device and base station can be moving relative to a fixed reference point such as the earth.
Such wireless communication systems typically are full duplex systems where communication can occur in a downlink transmission (i.e., transmission from the base station to the user device) and an uplink transmission (i.e., transmission from the user device to the base station) simultaneously. Full duplex systems can be either frequency division duplex (FDD) systems or time division duplex (TDD) systems. In a FDD system, the downlink and the uplink transmissions communicate over two separate frequency bands and associated center frequencies. Alternatively, in TDD systems, the downlink and uplink transmissions communicate over a single frequency band and associated center frequency. However, the uplink and downlink communications are separated in time. In a full duplex communication system, both the uplink signal and downlink signal must remain active. If either the uplink or downlink signal is lost for a significant period of time, the full duplex connection will fail.
A receiver in a user device typically derives the downlink frequency, which includes a Doppler shift if the user device is moving relative to the base station, by locking on to the received downlink signal from the base station. The user device will typically introduce this Doppler shift into the uplink signal it transmits towards the base station. As a consequence, the uplink signal received by the base station may experience a Doppler shift that is twice that measured by the user device.
User devices can typically handle relatively large Doppler shifts, for example, several kHz. However, base stations generally cannot handle such large Doppler shifts. For example a base station may only be able to handle a Doppler shift of up to 500 Hz. This may be due to the fact that base stations must be designed to handle signals from multiple user devices with multiple Doppler shifts. As a result of this design requirement, base stations may not generally be capable of detecting and adjusting to such relatively large Doppler shifts. Problematically, if the Doppler shift from a user device is outside of a base station receiver's capability, the base station will fail to decode the signals from the user device and the communication link will break down.
This problem is exacerbated as the relative speed between a user device and base station increases, along with an associated increase in Doppler shift. For example, in an LTE-based air-to-ground communication system, the user devices are located in airplanes that move at speeds that are several times higher than the typical speed limits allowed by commercial LTE systems.
Accordingly, there is a need for a system and method that can dynamically detect and estimate Doppler shifts in a wireless communication system. Moreover, there is a need to detect, estimate and compensate for Doppler frequency shifts on either, or both, the uplink transmission signal and the downlink transmission signal.